I guess it is, if you have a certain conception of your job.
A tenure-track scholar should not think that way, though. You are hired because of your competence in teaching certain classes. This competence is based on your scholarly expertise and training. You should have classes you already know how to teach, and then the intellectual wherewithal to develop new courses. If you feel the compulsion to prepare every single aspect of the course (each lecture, each assignment) before the semester starts, then you will free up an enormous amount of time during the semester. All you will have to do is show up and do the grading, with the rest of your time free for research or service, or for developing next semester's courses. If you just want to do a basic syllabus and prepare every week, then you will have less time during the semester for other things, but you won't have to spend as much time before. If you've been teaching for a while, you will have a combination of updating existing courses and developing new ones.
You probably won your first t-t job in competition with other people. One thing that got you the job was the idea that you had a certain number of courses "in you" already. A survey in your own field; a composition class, etc... You might have to present a sample syllabus or two.